The present disclosure relates generally to the field of electrical switch control systems. More specifically, the present disclosure relates to systems and methods for controlling a switch to selectively connect a power source e.g., a three phase, medium voltage power source) to one or more capacitors.
Switched capacitor banks are installed on poles and at sub stations to apply power factor correction (e.g., by altering the load phasing) to the power grid in response to the application and removal of heavy industrial inductive loads such as motors. When loads are not in phase, additional reactive currents increase transmission losses which results in wasted energy and the need for additional generating capacity. In some systems, a separate control senses the voltage-to-current phase relationship and commands the capacitor switch to open and close based on the relationship. Applying capacitors may help improve the transfer efficiency of the electrical energy being transmitted through the power grid. Medium voltage applications (e.g., 5 kV-38 kV) often include capacitors that are switched on and off based on power factor correction needs.
If a switch closes at a time when the AC voltage across the switch is not at a waveform zero, disturbances may occur due to heavy inrush currents as the capacitors are charged. The disturbances may include, for example, voltage dips, harmonics, resonance peaks and/or other undesirable effects on the electrical system. Such disturbances can cause problems with sensitive customer equipment, such as industrial VFD (variable frequency drive) motor controllers. Due to the mechanical and electrical complexity, the majority of medium voltage capacitor switches close randomly with respect to voltage. Some systems are configured with a resistor inserted in series with the switch to charge the capacitor to voltage, reducing the inrush current. Such systems may be acceptable for some applications, but may not perform in an acceptable fashion for more sensitive applications.
Controllers that are configured to close when the voltage across the capacitor switch is nearly zero volts are typically complex, expensive, and difficult to commission/install because they must handle a complex mixture of mechanical and electrical variations. Complex algorithms may be used to estimate the voltage across each switch, and such algorithms may require the installer to provide detailed information about the installation, such as the phase rotation (e.g., A-B-C phasing or A-C-B phasing), Wye/Delta capacitor connections and capacitor grounding (e.g., grounded or ungrounded). Some controllers blindly time their operations based on a single phase voltage sensor and calibration information regarding the electrical system to which the system is connected. For example, a voltage sensing transformer may reference only phase A of a three phase system. If the capacitor bank is connected in a grounded Wye configuration, it is expected that the electrical timing between zero volts of each phase is separated by 120 electrical degrees. The phase rotation must be known to configure such a controller.
Additionally, conventional zero-closing switches are configured to measure voltage on a single side of the switch (e.g., the power source side). When a Medium Voltage AC switch operating a capacitor opens, the current is cleared at a zero crossing. Since the current and voltage signals are out of phase 90 degrees due to the capacitor, a near peak trapped DC charge is left on the capacitor after the switch is opened. Capacitors have an internal resistor that is configured to slowly dissipate this energy until the voltage across the capacitor is brought to zero volts. In order to ensure that the capacitor has fully discharged (e.g., such that the voltage on the capacitor side of the switch is zero) and that closing the switch will not induce abnormally large inrush current (e.g., more than 6 times load capacitive load current), conventional zero-closing switches may be configured to wait a predetermined amount of time (e.g., five minutes) after the switch was last opened before closing the switch again. Closing the switch prior to the predetermined amount of time may produce an abnormally large inrush current (e.g., up to 80 times load current) as the source voltage meets a large trapped charge voltage on the capacitor. Specialized interlocking control equipment, training, and/or signage is often used to prevent closing of the switch prior to the passage of the predetermined amount of time.
There is a need for an improved control system for controlling the operation of switches used to selectively connect power sources to switched capacitors. There is also a need for a control system that is highly repeatable under a variety of environmental conditions. Further, there is a need for a control system that can be connected to a variety of different power system and/or capacitor configurations without the need for a substantial amount of specialized calibration to the individual types of configurations. Further still, there is a need for a control system that provides greater knowledge and awareness of the voltage conditions on both sides of the switch. There is also a need for a control system that does not require the switch to wait a predetermined amount of time after opening before the switch may close again.